1. Field of the Invention
The present invention relates to an improvement to connections for rigid or flexible shafts, such as tubes or cables, having a reinforcement framework withstanding at least the tractive forces, said framework being formed of threads free to move with respect to each other over the part of the shaft external of the connection, as well as the method for producing such connections.
Such shafts may be fluid transfer tubes, such as oil effluents.
The invention is particularly well adapted to shafts subjected to high internal pressures and to fatigue due particularly to the differential pressure, traction or flexion variations. It applies to all kinds of framework thread materials such as metal, Kevlar, polyamide, glass or carbon fibers impregnated with stabilizable plastic material, disposed in layers of parallel threads or in braids.
By stabilizable plastic material is meant substances able to assume a stable form by a physical or chemical process such as fusion with cooling or cross-linking. These materials may, for example, be thermo plastic materials, heat hardenable materials, elastomers or metals.
2. Description of the prior art
The transfer of forces from one shaft section including free frameworks to the next section or to an anchorage point is achieved by means of couplings. There exist numerous types of couplings. Some couplings are called "drawn", others are called "wedged", finally others are called "plugged".
Drawn couplings use, at least for the traction framework, the embedment of the threads of this framework in a block of malleable material, this block is formed and self embedded in the conical internal form of the coupling by the so called wire drawing operation.
Wedge couplings provide the anchorage of at least the traction framework by jamming its threads between hard jaws which interfit.
Plugged couplings provide the anchorage of at least the traction framework by jamming, in an internal conical form of the coupling, a plug in which the ends of the framework threads are embedded, this plug may be obtained by casting, by injection, or by any other means for impregnating the threads. The triaxial compression of the plug in the conical shape provides holding of the threads. The products used for plugging are materials which are liquid at the time of shaping, solid and resistant to deformation at the time of use, such as metals or heat hardenable resins.
Whether the shafts are rigid or flexible, cables or tubes, the performances of these couplings are rarely equal to unity. That is to say that the cable or tube breaks flush with the coupling or even thereinside, for forces which rarely reach the value calculated from the proper strength of the strands or threads. As a general rule, the tube breaks at the level of the coupling for forces which are rarely greater than two thirds of the above mentioned calculated value.
Thus, it is only after a long period of static service that flexible cables or flexible tubes break in the current length, especially if they are subjected to bending. The current length of a shaft corresponds to the zone of the shaft insensitive to the influence of the couplings which it has at its ends. For a shaft of diameter D, the current length begins and finishes at about 2 to 3D from the flush position of each of the couplings. In somes cases, however, using resin plugged couplings, in suitably dimensioned cones, the efficiency is equal to unity and the breakage is obtained in the current length.
Despite everything it is rare for this favorable behavior to be maintained when the flexible shaft thus formed is subjected to alternating flexion or pulsed pressure fatigue, this fatigue resulting in breakage flush with the couplings, when it has not been previously induced in the current length by corrosion, wear or seizure phenomena.
The couplings plugged more particularly with resin do not damage the ends of the threads of the framework and allow shafts to be used under higher tractive forces. However, the dynamic attenuation phenomena of the performance of plugged couplings leads more particularly to reducing the load exerted on these couplings and, consequently, the other types of couplings are often preferred, although not having the same efficiency in use, for their efficency is then maintained during the whole lifespan of the equipment. The initial lower strength is thus maintained intact for a longer time.
With the device and method of implementation of the invention, an efficiency equal to unity for resin plugged couplings may be kept for periods equal to the lifespan of the flexible or rigid shaft, and the efficiency of drawn couplings and wedge couplings may be improved while maintaining their useful lifespan.